Ba-enrichment in phlogopite of a nephelinite from Bohemia
Seifert, Wolfgang; Kämpf, Horst
European Journal of Mineralogy Volume 6 Number 4 (1994), p. 497 - 502
published: Jul 27, 1994
manuscript accepted: Apr 20, 1994
manuscript received: Apr 15, 1993
ArtNo. ESP147050604002, Price: 29.00 €
Abstract Ba-rich phlogopite occurs in nephelinite from Komorni hurka (Kammerbuhl), Czech Republic. The mica usually forms euhedral microphenocrysts and subhedral groundmass crystals. Its BaO content ranges from 13 to 16 wt%, at about 5 to 6 wt% TiO2. The average mineral formula is (Ba0.48K0.46Na0.04(Ca0.02)(Mg2.22Fe0.40Ti0.32Mn0.01)[(Si2.54Al1.46)O10](F1.39O0.61). It shows the presence of a considerable "oxy-component". Rarely, phlogopite of markedly lower Ba and higher Ti contents was found included in olivine. This mica was probably formed earlier, under higher temperatures and Ti contents in the melt, than the more Ba-rich, relatively Ti-poor phlogopite. The Komorni hůrka mica is comparable to Ba-rich micas from several other localities. In particular, the present report complements the study of Edgar (1992), concerning Ba-rich micas in nephelinitic rocks from the West Eifel, Germany. The high Ba contents in phlogopite from Komorni hürka contrast with the Ba-poor phlogopite from the neighbouring Zelezná hürka (Eisenbühl) volcano, although the host rocks have similar geochemistry, age, and tectonic position. Differences in mica composition between nephelinites from both volcanoes are not a question of the amount of Ba supplied by the melt, but of different petrogenetic conditions.